Cryogenic freezing system

ABSTRACT

The operation of a prior art freezer installation for freezing food can be improved by: 
     (1) providing air curtains at the doors of the freezer; 
     (2) using a pulse bag filter for separating ice from the air leaving the freezer; and 
     (3) ensuring that the air leaving the freezer is colder than -80° F.

The present invention relates to systems for freezing articles in afreezer using a circulation system of air at cryogenic temperatures. Itis particularly concerned with a more efficient circulating airrefrigeration system for large scale food freezing applications.

BACKGROUND OF THE INVENTION

Refrigeration systems employing air at cryogenic temperatures forfreezing food are commercially available, for example, see U.S. Pat.Nos. 3,733,848 and 3,868,827. In the latter patent, air is compressed ina first stage compressor, cooled in an intercooler, further compressedin a second stage compressor, cooled in another intercooler, furthercooled by countercurrent exchange with cold air leaving the freezer andfinally expanded in an expansion turbine mechanically coupled to thesecond stage compressor where the gas is reduced to about -180° F.before being directed into the freezer. This prior art freezer has beenused in combination with a vortex separator for removing particles ofice in excess of 5 microns in diameter from the air leaving the foodfreezer. However, despite this separation ice has been found to build upin the regenerative heat exchanger and to result in an intolerablepressure drop across the main heat exchanger which greatly reduces theefficiency of this system.

SUMMARY OF THE INVENTION

To overcome the disadvantages of the prior art cryogenic freezers, thepresent novel system affords greater efficiency by replacing the vortexseparator with a pulse bag filter to maintain the temperature of the airleaving the freezer at temperatures of below -80° F. Additionalefficiency is achieved by supplying dry air to maintain an air curtainat the inlet and outlet of the freezer to prevent entry of warm, moistatmospheric air into the freezer and subsequent refrigeration loss.Finally, air is introduced into the bottom of the freezer adjacent tothe freezer inlet to promote more rapid freezing of the food articles,thus limiting the dehydration of the food products and the ice formedtherefrom.

In accordance with one embodiment of the present invention, but notrestricted thereto, the novel system which is used for refrigeration,particularly in the rapid freezing of food products comprises a freezerhaving an inlet for admitting the articles to be frozen, an outlet forpermitting the frozen articles to leave said freezer and a conveyor fortransporting the articles through the freezer from the inlet to theoutlet thereof; a refrigerant supply main connected to the freezer forintroducing refrigerant air to the freezer; a return main connected tothe freezer for receiving the warmed air from the freezer; refrigerationmeans connected to the refrigerant supply main for supplying the supplymain with air at cryogenic temperatures; a main heat exchanger having ahigh pressure side connected to the refrigeration means and a lowpressure side connected between the refrigeration means and the returnmain for exchanging the refrigerant value of the warmed air from thereturn main in the low pressure side with the air from the refrigerationmeans in the high pressure side; and bag filter means connected betweenthe return main and the main exchanger for removing ice particles fromthe warmed air in said return main prior to exchanging its refrigerationvalue in the main heat exchanger. The filter means comprises at leastone bag which is periodically pulsed to remove ice collected on theoutside thereof through the pulsing action of a portion of the airleaving the high pressure side of the main heat exchanger prior toentering an expansion portion of the refrigeration means.

The refrigeration means comprises three stages of compression and oneexpansion turbine. The air in the system is progressively compressed bythe three compressor stages, is cooled in the main heat exchanger and isexpanded in the expander of the refrigeration means before beingintroduced into the freezer.

The air that is withdrawn from the freezer is passed through the bags ofthe pulse bag filter to remove ice particles in excess of approximately1 micron in diameter, is warmed in the main heat exchanger and isrecycled through the refrigeration means.

The present apparatus and method thereof enable one to recover air fromthe exit of the freezer at temperatures colder than -80° F.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic flow diagram illustrating a preferred embodimentof the present invention; and

FIG. 2 is a more detailed flow diagram illustrating the preferredembodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the FIGURES, FIG. 1 illustrates the combination offreezer 1, refrigeration unit 2, main heat exchanger 3, make-up airsystem 4 and filter 5 comprising bag 6 and rotary valve 7. The detailsof the operation of filter 5 are described below.

In FIG. 2, freezer 1 is shown having feed station 8, discharge station 9and continuous belt conveyor 10 for transporting the articles to befrozen through the freezer. One suitable freezer for use in combinationwith the present invention is described in pending U.S. patentapplication No. 199,130 filed Oct. 22, 1980, assigned to Air Productsand Chemicals, Inc., the assignee of the present application.

Air at approximately 1 atmosphere and -205° F. is introduced fromrefrigerant supply main 12 through line 13 into freezer 1. The flowthrough line 13 is controlled by temperature indicating controller 15which controls valve 17. Air at approximately -100° F. exits freezer 1through line 20 connected to return main 22. The flow of the air fromfreezer 1 is controlled through valve 24 by means of pressure indicatingcontroller 26.

Air in return main 22 is passed to bag filter 5 via line 28 where theparticles of ice formed from the food products being frozen areseparated from the air and is designed to extract 99.9% of all theparticles having a diameter of 1 micron or greater through valve 7. Itis understood that for particles that are not substantially spherical,particles having their maximum dimension equal to 1 micron or greaterare removed from the air in filter 5.

The substantially ice-free air from filter 5 is then passed through line30 to main heat exchanger 3. Specifically the air in line 30 passesthrough low pressure side 31 of main heat exchanger 3 where it is heatedto about 95° F. at a pressure of about 10.7 psia. Air from main heatexchanger 3 flows through line 34 to refrigeration unit 2. Although theapparatus of this invention comprises the use of any combination of 3stages of compression and an expander, one form of refrigeration unit 2that is preferably used in this embodiment illustrated in FIG. 2 iscommercially available under the designation of TA-100 3-StageCentrifugal Compressor from Joy Manufacturing Company, which compressorhas an expansion turbine mounted on the open end of its second postthereof. This refrigeration unit comprises motor 38 which rotates gear40 which in turn rotates first post 42 and second post 43. Firstcompressor stage 46 is mounted on one end of first post 42 and secondcompressor stage 47 is mounted on the other end thereof. Thirdcompressor stage 49 is mounted on one end of second post 43 and expander50 is mounted on the other end thereof. Air is compressed in firstcompressor stage 46, passed through line 51, cooled in intercooler 52and compressed in second compressor stage 47. The air is then passedthrough line 53, cooled in intercooler 54, and compressed in thirdcompressor stage 49 to 78 psia. The air is passed through line 55 andcooled to about 100° F. in aftercooler 56. The compressed air from thecompressor stage of refrigeration unit 2 is passed through line 57 andcooled to about -95° F. in the high pressure side 58 of main heatexchanger 3. The major portion of the cold air from high pressure side58 is passed through line 59 and expanded in expander 50 before beingpassed into refrigerant supply main 12 via line 60. From main 12, therefrigerant air is introduced into freezer 1 at a temperature of about-205° F. and 15 psia via line 13.

The flow of air through main heat exchanger 3 is equalized by means ofvalve 62 and pressure indicating controller 63.

Referring to FIG. 1, air in line 28 enters filter 5 through inlet 64 andis deflected by deflector 65. The largest particles of ice fall directlyinto the conical bottom or hopper or filter 5 for removal through solidsdischarge spout 66 and rotary valve 7. The air stream 67 flows upwardthrough at least one bag 6 mounted on clamps 68 and supported by cage69. The ice particles remaining in the air having a diameter or largestdimension of at least 1 micron are collected on the outside of bag 6.The ice-free air passes through outlet 70 and through line 30 to mainheat exchanger 3. Periodically bag 6 is pulsed by means of a small sidestream, approximately 1 to 5% by weight, of high pressure air (about 75psig) which is directed through line 71 to a position directly aboveventuri nozzle 72. The pulse of air stops the flow of substantiallyice-free air, i.e. the air contains no more than 0.1% by weight of iceafter filtration, and the pulse causes a shock wave to travel down bag6. This wave forces bag 6 to momentarily depart from wire cage 69 toposition 73 (shown in phantom), to snap back in place and to dislodgethe ice built up on the outside of bag 6 into the hopper of filter 5.The composition of the bag can be of any suitable material, for example,a polyester felt bag coated with Teflon® polymer. A suitable bag filterfor this embodiment is commercially available as P-1-120 Pulse DustCollector, which is a single width unit containing 120 bags; seeBulletin AP-750 entitled "Buffalo AEROTURN® Pulse Dust Collector Type P"from Buffalo Forge Company, Buffalo, New York, September 1977 forfurther details of this device, the description of which is incorporatedherein by reference.

The temperature of the cold air leaving high pressure side 58 is about-95° F. or within a few degrees of the air entering filter 5 from line28. The latter is combined with the small side stream used in thepulsing action described above to form a stream at about -100° F.entering low pressure side 31 of heat exchanger 3. All of the exteriorsurfaces of filter 5 are provided with suitable insulation 74 to preventheat loss of this stream in filter 5.

Referring again to FIG. 2, ingress of moist air into freezer 1 isinhibited and heat loss is prevented by air curtains 76 and 77 which arepositioned above inlet 8 and outlet 9, respectively. One acceptableversion of an air curtain is commercially available as Transvector® AirFlow Amplifier. Alternatively, a venturi system powered by the smallcompressed air flow from make-up air system 4 through lines 78 and 79can be utilized for this purpose.

Dry make-up air for the cryogenic refrigeration system and for the aircurtains is provided by air in line 80, compressed in compressor 81 anddried in drier 82 containing a suitable dessicant such as alumina or 5Amolecular sieves. The dried compressed air is combined via line 84 withair in line 57 to high pressure side 70 of main heat exchanger 3.

It is obvious from FIG. 2 that supply main 12 and return main 22 extendin both directions such that additional freezers beyond the singlefreezer 1 shown may be used provided that sufficient refrigerationcapacity is available from unit 2. Alternatively, additionalrefrigeration units identical to unit 2 may be connected to the supplyand return mains.

In order for refrigeration unit 2 to operate efficiently, the compressedair leaving aftercooler 56 must be cooled as effectively as possible inmain heat exchanger 3. This is achieved by inhibiting the build up ofice in main heat exchanger 3 by: (1) the provision of air curtains 76and 77, (2) bag filter 5 and (3) maintaining the air leaving freezer 1colder than -80° F. Most of the ice is likely to form on high pressureside 58 of main heat exchanger 3. Using the process of the presentinvention the rate of icing is reduced by a factor of roughly 5, if thetemperature of air leaving freezer 1 is -100° F., when compared with therate of icing in cryogenic freezers of the type described above inconnection with a discussion of the prior art.

What is claimed is:
 1. In an apparatus for freezing articles comprisingin combination a freezer having an inlet for admitting the articles tobe frozen, an outlet for permitting the frozen articles to leave saidfreezer and a conveyor for transporting the articles through saidfreezer from said inlet to said outlet thereof; a refrigerant supplymain connected to said freezer for introducing refrigerant air to saidfreezer; a return main connected to said freezer for receiving thewarmed air from said freezer; refrigeration means connected to saidrefrigerant supply main for supplying said supply main with air acryogenic temperatures; and a main heat exchanger having a high pressureside connected to said refrigeration means and a low pressure sideconnected between said refrigeration means and said return main forexchanging the refrigerant value of said warmed air from said returnmain in said low pressure side with the air from said refrigerationmeans in said high pressure side; the improvement which comprises;bagfilter means connected between said return main and said main exchangerfor removing ice particles from the warmed air in said return main priorto exchanging its refrigeration value in said main heat exchanger, whichfilter means comprises at least one bag which is periodically pulsed toremove ice collected therein.
 2. The apparatus of claim 1 wherein theice particles are removed in said bag filter means through the pulsingaction of a portion of the air leaving the high pressure side of saidmain heat exchanger and have a diameter of 1 micron or greater.
 3. Theapparatus of claim 1 wherein air is introduced into said freezer at orcolder than -180° F. and is withdrawn from said freezer colder than -80°F.
 4. The apparatus of claim 1 wherein means are provided for supplyinga curtain of dry air over said inlet and said outlet to control heatloss from said freezer.
 5. In a method for cryogenically freezingarticles comprising the steps of:(a) contacting articles in a freezerwith air at a temperature of about -180° F. or colder; (b) continuouslywithdrawing a portion of the warmed air from the freezer aftercontacting the articles within the freezer with the air; (c) alternatelycompressing the warmed air from step (b) and cooling the compressed airstream to a temperature substantially above - -180° F.; (d) exchangingthe refrigeration valve of the air from step (b) with at least a portionof the compressed air stream from step (c); and (e) expanding thecompressed air stream from step (d) to cool it to a temperature ofabout - -180° F. or colder for use in said freezer; the improvementwhich comprises passing the warmed air from step (b) through a bag of abag filter and collecting the ice particles on the outside of the bag.6. The method of claim 5 wherein the ice particles collected on the bagof said bag filter have a diameter of one micron or greater.
 7. Themethod of claim 5 or 6 wherein a small portion of the compressed airstream from step (d) is periodically passed to said bag filter todislodge the ice collected on the bag.
 8. The method of claim 5 or 6wherein at least 99.9% by weight of the ice particles are removed fromthe warm air leaving the freezer.
 9. The method of claim 5 or 6 whereinstep (c) comprises 3 stages of compression with intercooling betweeneach stage.